Often when doing genealogical or family history research in the United States you want to know if there are manuscript materials available. In the past this was done using the National Union Catalog of Manuscript Collections in a printed series of several hundred volumes or by going to the online site at http://www.loc.gov/coll/nucmc/
OCLC has now opened up one of it's proprietary databases, ArchiveGrid, to the public, http://beta.worldcat.org/archivegrid/
With ArchiveGrid you can share your location with OCLC and it can pinpoint archive collections near you. You can also enter the location details or the postal zip code for a location to get links to archives in an area of interest to your genealogy.
Hopefully this will remain a free resource after it exits beta testing.
Persons with ancestry in the United Kingdom can use the Access to Archives, A2A, site at https://www.nationalarchives.gov.uk/a2a/